Oligosaccharides are food products with interesting nutritional properties. They may be naturally present in food, mostly in fruits, vegetables or grains, or produced by biosynthesis and/or purification from natural sugars or polysaccharides and added to food products because of their nutritional properties. Resistance to enzymatic reactions that occur in the upper part of the gastrointestinal tract allows oligosaccharides to become ‘colonic nutrients’ for gut microbiota, as some resident intestinal bacterial species express enzymes that enable oligosaccharide harvest and metabolism by fermentation. Oligosaccharides that selectively promote the growth of a bacterial species of interest and thus equilibrate intestinal microbiota may be referred to as prebiotics.
The normal microbiota of humans is exceedingly complex, and varies by individual depending on genetics, age, sex, stress, nutrition and diet of the individual. It has been calculated that a human adult houses about 1012 bacteria on the skin, 1010 in the mouth, and 1014 in the gastrointestinal tract. The latter number is far in excess of the number of eucaryotic cells in all the tissues and organs which comprise a human.
The microbiota of the gut perform many metabolic activities, and influence the physiology of the host. Bacteria make up the majority of the gut microbiota, although it includes anaerobic members of archaea and eukarya. The majority of these microbes are obligate anaerobes, and a small percentage facultative anaerobes. It is estimated that between 300 and 1000 different species live in the gut, however, it is known that a smaller number of species dominate. Most belong to either the Firmicutes or Bacteroidetes phyla. Common genera include: Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, Akkermansia, Faecalibacterium, Roseburia, and Bifidobacterium. Species from the genus Bacteroides alone constitute about 30% of all bacteria in the gut, suggesting that this genus is especially important in the functioning of the host.
Without gut microbiota, the human body would be unable to utilize some of the undigested carbohydrates it consumes, because some members of gut microbiota have enzymes that human cells lack for breaking down certain polysaccharides. Carbohydrates that humans cannot digest without bacterial help include certain starches, fiber, oligosaccharides and sugars that are not digested and absorbed in the upper portion of the GI tract, e.g. lactose in the case of lactose intolerance and sugar alcohols, mucus produced by the gut, and many types of complex dietary plant polysaccharides. Bacteria turn carbohydrates they ferment into short chain fatty acids, or SCFAs. These materials can be used by host cells, providing a major source of useful energy and nutrients for humans. SCFAs increase the gut's absorption of water, reduce counts of damaging bacteria, increase growth of human gut cells, and potentiate the growth of indigenous syntrophic bacteria. Evidence also suggests that bacteria enhance host absorption and storage of lipids. Changing the numbers and species of gut microbiota can alter community function and interaction with the host.
Human breast milk contains several different classes of molecules that perform numerous biological roles for the nursing infant, including providing calories and other nutrients. One of the most abundant classes of molecules in human milk is the milk oligosaccharides, a family of ˜200 structurally related carbohydrates. When consumed by the infant milk, milk oligosaccharides pass to the distal portion of the digestive tract undigested. Milk oligosaccharides are able to serve as a carbon and energy source for the developing consortium of microbes that assemble in the infant intestine shortly after birth.
Infant formula has been widely used throughout the world as a substitute for mothers' milk to feed infants. One of the major challenges of formulating a synthetic food for infants is mimicking the properties of human milk. While simple molecules such as lactose, amino acids/proteins, and vitamins are readily available for addition to formula, some important bioactive molecules, such as milk oligosaccharides, are not readily obtained in large quantities, and therefore are not typically added.